Bonded fibrous materials

ABSTRACT

A composite material comprises inorganic-bonded alkaline earth silicate fibers in which any bonding agents or fillers comprise low amounts of aluminum so that the composite material comprises less than 1% by weight aluminum expressed as Al 2 —O 3 .

This invention relates to bonded fibrous materials and is particularlyapplicable to materials comprising saline soluble fibres bonded with abinder.

Refractory ceramic fibres (RCF) are well known materials and typicallycomprise an alumino-silicate inorganic fibre formed from an oxide meltwhich is spun, blown, drawn, or otherwise formed into fibres. Such RCFfibres are used in the manufacture of various industrial and domesticarticles. Typical uses of RCF are for applications in which resistanceto temperatures in excess of 800° C. is required.

Much RCF fibre is used in the form of needled blankets of fibre in whichstructural integrity is provided by the fibres that are tangled togetherin the needling process. (Such products are known as “blanket”).Sometimes a binder is used to lock the fibres together subsequent toexposure to high temperature. Blanket can be processed further to formcut shapes or folded to form insulating modules.

RCF fibre is also used in the production of so-called “ConvertedProducts”. Converted products comprise materials in which the RCF isprocessed further to provide materials in which the RCF is present aseither a minor or major constituent. Typical converted products includethe following:—

-   “Board”—substantially rigid flat sheets containing inorganic and/or    organic binders produced by a wet process (for example made by    dehydrating a suspension of RCF and binders);-   “Paper”—a flexible fibrous insulating material with a thickness of    less than or equal to 6 mm, formed on paper making machinery (for    example RCF in sheet form with a binder);-   “Shapes”—substantially rigid shapes made of ceramic fibre with the    addition of inorganic and/or organic binder, fired or unfired (for    example, RCF formed by vacuum forming into a variety of shapes);-   “Fire shapes”—RCF formed by a vacuum forming route and used for    domestic and industrial fires either as radiant bodies or for    decorative appearance;-   “Castables”—ceramic fibre with inorganic and/or organic binder which    may be cast (for example, RCF in the form of cements, concretes and    mortars);-   “Mastics”—A mouldable material containing RCF with binders and which    may be trowelled, hand moulded, or dispensed from a pressure gun and    which sets upon drying/heating;-   “Extrusion”—A mastic-like material that may be used in the    manufacture of extruded sections and tubes;-   “Textiles”—ceramic fibre which has been woven with or without the    addition of other filaments, wires, or yarns (for example, RCF    formed into rope, yarn, mats and the like by textile technology).

In many of the above mentioned applications binders are used. There aretwo broad classes of binders:—

-   “Organic binders”—which serve to improve the handling    characteristics of the product concerned at low temperatures but    which burn off at higher temperatures. Organic binders include, for    example, such materials as starch.-   “Inorganic binders”—which may be effective to improve the handling    characteristics of the product concerned at low temperatures, but    which also give integrity to the product after exposure to high    temperatures. Inorganic binders include, for example, such materials    as colloidal silicas, aluminas, and clays.

All of the above materials and concepts are well known in the refractoryindustry.

Although extremely useful, RCF is an inorganic fibrous material.Inorganic fibrous materials can be either glassy or crystalline.Asbestos is an inorganic fibrous material one form of which has beenstrongly implicated in respiratory disease.

It is still not clear what the causative mechanism is that relates someasbestos with disease but some researchers believe that the mechanism ismechanical and size related. Asbestos of a critical size can piercecells in the body and so, through long and repeated cell injury, have abad effect on health. Whether this mechanism is true or not regulatoryagencies have indicated a desire to categorise any inorganic fibreproduct that has a respiratory fraction as hazardous, regardless ofwhether there is any evidence to support such categorisation.Unfortunately, for many of the applications for which inorganic fibresare used, there are no realistic substitutes.

Accordingly there is an industry and regulatory demand for inorganicfibres that will pose as little risk as possible (if any) and for whichthere are objective grounds to believe them safe.

A line of study has proposed that if inorganic fibres were made thatwere sufficiently soluble in physiological fluids that their residencetime in the human body was short; then damage would not occur or atleast be minimised. As the risk of asbestos linked disease appears todepend very much on the length of exposure this idea appears reasonable.Asbestos is extremely insoluble.

As intercellular fluid is saline in nature the importance of fibresolubility in saline solution has long been recognised. If fibres aresoluble in physiological saline solution then, provided the dissolvedcomponents are not toxic, these fibres should be safer than fibres thatare not so soluble. Accordingly, in recent years, a number of differenttypes of fibre have been proposed which are refractory and yet solublein body fluids. Such fibres comprise alkaline earth silicates (e.g.WO87/05007, WO89/12032, WO93/15028, WO94/15883, WO96/02478, andWO97/49643) which are soluble to varying extent in body fluids.

A problem with saline soluble fibres is that by their nature they aremore reactive than RCF and therefore cannot always be used as a directreplacement for RCF. The applicants have found that one aspect of thisreactivity is that the performance of materials at temperatures inexcess of 1100° C. is extremely adversely affected by the presence ofaluminium in the binders and fillers conventionally used with RCF. Theapplicants speculate that this adverse effect is due to a eutecticcomposition that has been reported to lie at about 1238° C. in theCaO—Al₂O₃—MgO—SiO₂ phase field.

The applicants have further found that sodium and boron badly affectperformance of fibres above 1200° C.

Accordingly the present invention provides a composite materialcomprising bonded alkaline earth silicate fibres in which any bondingagents or fillers comprise low amounts of aluminium so that thecomposite material comprises less than 1% aluminium expressed as Al₂O₃.Preferably the composite material comprises less than 0.5% by weight ofaluminium expressed as Al₂O₃. More preferably the composite materialcomprises less than 0.1% by weight of aluminium expressed as Al₂O₃. Yetmore preferably the composite material is essentially free of aluminium.

In a further feature the composite material comprises less than 1%,preferably less than 0.5%, more preferably less than 0.1% by weight ofsodium expressed as Na₂O and is still more preferably essentially freeof sodium.

In a still further feature the composite material comprises less than0.5% by weight of boron, preferably less than 0.1% of boron expressed asB₂O₃.

Further features of the invention are apparent from the claims and thefollowing description, which refers to various applications in which theinvention is applicable.

Insulation Board and Shapes.

The invention can be illustrated in its broadest concept by reference toTable 1 which indicate the results of making board using alkaline earthsilicate fibres of the SUPERWOOL™ 612™ composition (available fromThermal Ceramics de France SA or Thermal Ceramics Limited). Such fibreshave a nominal composition (by weight) of SiO₂ 64%, CaO 17%, MgO 13.5%,ZrO₂ 5%, and impurities <0.5%, and are usable at temperatures in excessof 1200° C. and up to 1250° C.

Boards and some shapes are conventionally made by first formulating asuspension of fibre with a cationic organic binder such as a starch andan anionic inorganic binder such as colloidal silica. The cationicorganic binder and anionic inorganic binder flocculate, drawing thefibre into a flocculated suspension.

The suspension is placed in contact with mesh moulds and vacuum isapplied to the moulds to vacuum form articles either on the outside ofthe mesh (male mould) or on the inside of the mesh (female mould).Vacuum is applied to the mould until a sufficient thickness of fibre hasbuilt up and the mould is then removed from the suspension, the vacuumremaining on for a little while to promote dewatering. This processproduces a wet green article containing about 50%–70% water.

At this stage the product is extremely fragile having the consistency ofwet cardboard. The wet green article is dried, for example at atemperature of about 150° C. and the organic binder then gives somehandling strength. Relatively low amounts of inorganic binder are usedin the formation of such materials. A typical recipe for use in vacuumforming would comprise 100 kg of fibre, 25 kg of colloidal silica (a 30%solution i.e. 7.5 kg dry weight), 6.5 kg starch and 1000 gallons(approximately 4500 liters) water. The silica in this formulationamounts to about 0.16% of the suspension formulation and about 7% of thedry materials.

When first fired by the end user of the shape or board the organicbinder burns off and the inorganic binder binds the fibres.

Recipes 1, 2 and 3 of Table 1 were tested in the discontinuousmanufacture of special shapes. As can be seen Recipe 1 of Table 1 meltsat 1250° C. due to the presence of aluminium in the clay. The aluminiumreacts with the CaO, MgO, and SiO₂ of the fibre to form a eutecticmixture. Although Recipe 1 failed at temperature Recipes 2 and 3 appearto give similar results. Recipes 4 to 10 were tested in the continuousmanufacture of board.

Recipe 4 refers to Table 2 to show the effect of aluminium compounds (asaluminium sulphate present in re-cycled wastewater) on high temperaturebehaviour. This appears to be extremely detrimental.

Recipes 5 and 6 show the effect of adding talc as filler. This appearsto improve modulus of rupture and compressive strength. Recipes 5, 7 and8 allow comparison with other filers, talc giving the best result.

Recipes 5, 9 and 10 allow comparison of the variation of the amount ofcolloidal silica. Recipe 9 appears the best.

TABLE 1 Recipe 1 2 3 4 5 6 7 8 9 10 Fibre 64.0%  73.3% 76.1% 80.3% 78.1%75.2% 78.1% 78.1% 76.0% 74.1% Clay  24% Talc 14.2% 15.0% 14.6% 17.5%13.2% 11.8% Wollastonite 14.6% Fumed Silica 14.6% Colloidal Silica 7.0%22.8% 6.3% 3.9% 3.9% 3.9% 3.9% 7.6% 11.1% (30%) Cationic Starch 5.0%3.9% 2.8% Starch 4.1% 2.9% 2.9% 2.9% 2.9% 2.8% 2.7% Xanthan gum 0.6%0.6% 0.5% 0.5% 0.5% 0.5% 0.4% 0.3% Density 313 320 277 303 316 320 304313 307 296 Modulus of rupture 1.4 0.69 1.2 0.9 0.75 0.87 0.78 0.73 1.261.22 by bending (MPa) M.O.R. at 1150° C. 0.04 M.O.R. at 1200° C. N.A.0.2 0.24 0.16 0.11 0.77 0.68 M.O.R. at 1250° C. 0.3 0.39 0.15 N.A. N.A.N.A. Loss on ignition at 5.2 4.5 4.2 3.6 3.6 3.6 3.4 3.4 3.4 3.6 800° C.Linear shrinkage see 2.1 2 2.3 1.9 2.6 2.6 24 h-1200° C. (%) TableLinear shrinkage melted 2.1 1.2 2 2.1 2.2 2.5 N.A. N.A. N.A. 24 h-1250°C. (%) Compressive 0.22 0.13 0.15 0.13 0.12 0.22 0.24 strength @ 10%(MPa) Compressive 0.1 Strength at 1150° C. for 10% (MPa) CompressiveN.A. 0.07 0.09 0.05 0.03 0.20 0.19 Strength at 1200° C. for 10% (MPa)

TABLE 2 Al₂O₃ content 10.2 6.7 6.3 5.1 0.4 (wt %) Shrinkage at melted1.8 1.7 1.6 1150° C. - 24 hours Shrinkage at melted glazed glazed glazed2.3 1200° C. - 24 hours

The recipes of Table 1 resulted in boards having the composition set outin Table 3.

TABLE 3 Recipe 1 2 3 4 5 6 7 8 9 10 Fibre 67.3% 87.2% 79.7% 80.3% 80.3%77.3% 80.3% 80.3% 80.3% 80.3% Clay 25.2% Talc 14.8% 15.0% 15.0% 18.0%13.9% 12.8% Wollastonite 15.0% Fumed Silica 15.0% Colloidal silica 2.2%8.2% 2.0% 1.2% 1.2% 1.2% 1.2% 2.4% 3.6% (30%) Cationic Starch 5.3% 4.6%2.9% Starch 4.1% 3.0% 3.0% 3.0% 3.0% 3.0% 3.0% Xanthan gum 0.6% 0.6%0.5% 0.5% 0.5% 0.5% 0.4% 0.3%

A typical and useful range of ingredients for making insulating boardand shapes by vacuum forming is (in weight percent):—

-   Alkaline earth metal silicate fibre 70–85%-   Colloidal silica (30% SiO₂ by weight) 3–25%-   Organic binder 1–6%-   Filler 11–20%    and from such ingredients typical and useful compositions in the    finished board are:—-   Alkaline earth metal silicate fibre 70–90%-   Colloidal silica (30% SiO₂ by weight) 1–10%-   Organic binder 1–6%-   Filler 11–20%

The examples given above have compositions in the range:—

-   Alkaline earth metal silicate fibre 77.3–87.2%-   Colloidal silica (30% SiO₂ by weight) 1.2–8.2%-   Organic binder 3.3–4.7%-   Filler 12.8–18%

In all of the above tested compositions the colloidal silica used wasNyacol™ 1430 which has a sodium content of about 0.4% by weight. Theamount of colloidal silica binder present was sufficiently low(3.9–22.8% by weight of the colloidal silica translating asapproximately 1.2–7 wt % silica binder in the finished product) that thesodium in the binder did not have an appreciable deleterious effect onthe properties of the material.

Papers

The same principles apply in the manufacture of papers. In theconventional manufacture of refractory paper slurry is made in likemanner to vacuum forming and is cast upon a wire former as in papermaking machinery.

The normal flocculant used is alum. The applicants have been able tomake refractory fibre paper using acrylic latex binders and an organicflocculant. Such papers have been tested to 1250° C. and whilecollapsing at 1200° C. the fibres remain in place providing someinsulating effect. In contrast, if alum is used as a flocculating agentthe paper melts.

A typical recipe (by weight percent) for the paper is:—

-   SUPERWOOL™ 612™ 90–95%-   Acrylic latex (PRIMAL HA8™ from Rohm & Haas) 5–10%-   Organic flocculants <1%

Suitable organic flocculants comprise the PERCOL L Series™ from AlliedColloids. These are polyacrylamide based products. In particular PERCOL230L works well.

Fire Beds, Artificial Coals, and Fire Shapes.

Articles that are directly exposed to flames are in an aggressiveenvironment with temperatures in excess of 1000° C. and exposure tocombustion products. Use of conventional binders with alkaline earthmetal silicate fibres (SUPERWOOL™ 612™) led to cracking of shapes. Theapplicants tested a series of compositions by making pieces usingdifferent colloidal silica binders each present at the same amount(about 6% by weight). These pieces were heated to 1000° C. for one hourand assessed for cracking, friability, and hardness (Shore ‘o’). Theresults of these tests are given in Table 4 below:—

TABLE 4 Specific Surface Na₂O Area Silica Colloid pH (wt %) (m²/g) (wt%) Cracking Friability Hardness Nyacol ™ 10.2 0.4 230 30 Bad Friable10–17 1430 fracturing Syton ™ 9.9 0.3 250 30 Some Fibrous 57 X30fracture Levasil ™ 9.5 0.17 200 30 Minor Friable 40 200-A-30 fractureBindzil ™ 9.7 0.42 220 40 Some Friable 40 40/220 fracture Bindzil ™ 9.5<0.1 220 30 Some Fibrous 47 30NH₃/220 fracture

From this it was deduced that:—

-   a) Alkaline pHs were associated with fractured pieces and could be    indicative of poor thermal shock resistance; and-   b) A reduction in Na₂O content appears to correlate with friability    of the product.

Accordingly, and in view of the growing perception that aluminium,sodium and boron are detrimental to the high temperature performances ofalkaline earth metal silicate fibres, the applicants requested theirsuppliers of colloid (Univar of Croydon, England—distributors for AkzoNobel) to supply colloidal silica meeting the following requirements notusually called for commercially:—

-   a) The colloidal silica should have a slightly acid to roughly    neutral pH, preferably in the range 6.5 to 7.5-   b) The soda content of the colloidal silica should be low,    preferably below 0.1 wt %-   c) The silica should not have appreciable amounts of aluminium    present.

A preliminary experimental product supplied under the reference Bindzil30/220LPN comprised 30 wt % silica, had a pH of 7.0 and comprised 0.08wt % Na₂O. The same trial as above was repeated using this silica and aproduct was produced which did not crack and remained fibrous with aShore ‘o’ hardness of 50. Further samples were made and subjected to a250 hours cycling test (2 hours on and 2 hours off under gas flame) andpassed this test.

Preliminary specifications for the typical colloidal silicas usable toachieve these results are:—

TABLE 5 SiO₂ content (by weight) 30 25 Viscosity <10 cP <10 cP pH6.5–8.0 6.5–8.0 Specific Surface Area (m²/g) 220–240 220–240 Density(g/cm³) 1.192–1.199 1.155–1.175 Na₂O content (by weight) <0.1  <0.1 

Such silicas are obtainable from Akzo Nobel under the reference Bindzil30/220LPN or the mark THERMSOL™.

A typical mixture for use in the manufacture of fire shapes comprises:—

TABLE 6 Fibre (e.g. SUPER WOOL 612 ™) 60 parts by weight Colloidalsilica (e.g. THERMSOL ™ = 12–14 parts by weight Bindzil 30/220 LPN [30%by weight SiO₂]) Starch (e.g. PLV available from Avebe, 2.5 parts byweight Netherlands)

The amount of water used in forming the slurry varies according toapplication but may for example range from 2700–4500 liters (600–1000gallons). The fibre typically represents about 0.5–4% by weight of fibrein water. Not all of the ingredients will be incorporated into a vacuumformed product formed from this mixture but typically such a mixtureleads to a product comprising approximately 6% by weight colloidalsilica, 3.5–5% starch with the balance fibre. The tolerable range forcolloidal silica is usually from about 4% to about 9% by weight in thefinished product.

Alternative compositions excluding organic binders (useful for such hightemperature applications as cooker rings) may be made for example fromslurry compositions 1 and 2 below:—

TABLE 7 Component COMPOSITION 1 COMPOSITION 2 “White water” 50–80% byvolume of 90–100% by volume of component 30% solids 30% solidsTHERMSOL ™ = THERMSOL ™ = Bindzil 30/220 LPN with Bindzil 30/220 LPNwith 10–0% by volume mains 20–50% by volume water mains water Fibre(SUPER- 0.5–4% by weight of 2–3% by weight of solids to WOOL ™ solids towhite water white water component 612) component

“White water” is the industry term for a mixture of water and colloidalsilica. Such slurry compositions lead to products comprising 15–30% byweight silica with the balance fibre.

Typical ring slurry compositions are, in parts by weight:—

Ring Slurry Composition 1

-   THERMSOL colloidal silica 355-   Fibre (SUPERWOOL™ 612) 3–5-   Fresh water 95    Ring Slurry Composition 2-   LEVASIL 200-A-40 colloidal silica 750-   Fibre (spun and chopped SUPERWOOL™ 612) 30-   Fresh water 250

LEVASIL 200-A-40 differs from LEVASIL 200-A-30, mentioned in Table 4above, in that in proportion to the amount of silica present LEVASIL200-A-40 has a lower amount of sodium. Additionally, and veryimportantly, LEVASIL 200-A-30 is aluminate modified whereas LEVASIL200-A-40 avoids alumina. LEVASIL 200-A-40 has the characteristics:—

-   Silica content (wt %) 40–41.5-   Na₂O content (wt %) 0.16–0.24-   Specific Surface Area (m²/g) 180–220-   pH 8.5–9.5.

The applicants find no deleterious effects in cooker ring production orperformance in using LEVASIL 200-A-40. Suitable slurry compositions forrings using a 40% colloidal silica are:—

TABLE 8 Component “White water” 65–100% by volume of 40% solids lowsodium con- component tent colloidal silica having a pH of less than 10with 35%–0% by volume mains water Alkaline earth metal 2–3 wt % byweight of solids to white water silicate fibres, for component examplechopped spun fibre

The materials described above under the heading “fire beds, artificialcoals, and fire shapes” (see Table 6) can also be used in widerapplications such as boards and shapes.

A typical composition for forming boards and shapes is, in parts byweight:—

-   Starch (Solvitose PLV) 4.8-   THERMSOL colloidal silica 32-   Fibre (SUPERWOOL™ 612) 80

Generally, the fibre content should preferably be between 0.5 and 5% ofthe weight of the water. Selection of particular compositions for thewide variety of applications that such bonded fibrous materials are usedin is a matter of experiment.

From the above results it can be seen that where the amount of binderused is high the amount of sodium in the binder is best kept low.Similar considerations apply for boron. It should be noted that somecolloidal silicas contain aluminium as a counter-ion and such colloidalsilicas should be avoided.

1. A composite material comprising colloidal silica-bonded alkalineearth silicate fibers in which any bonding agents or fillers compriselow amounts of alumina so that the composite material comprises lessthan 1% by weight aluminium expressed as Al₂O₃.
 2. A composite materialas claimed in claim 1 in which the composite material comprises lessthan 0.5% by weight of aluminium expressed as Al₂O₃.
 3. A compositematerial as claimed in claim 2 in which the composite material comprisesless than 0.1% by weight of aluminium expressed as Al₂O₃.
 4. A compositematerial as claimed in claim 1 in which the composite material isessentially free of aluminium.
 5. A composite material as claimed inclaim 1 and comprising less than 1% by weight sodium expressed as Na₂O.6. A composite material as claimed in claim 5 and comprising less than0.5% by weight sodium expressed as Na₂O.
 7. A composite material asclaimed in claim 6 and comprising less than 0.1% by weight sodiumexpressed as Na₂O.
 8. A composite material as claimed in claim 1 inwhich the composite material is essentially free of sodium.
 9. Acomposite material as claimed in claim 1 and comprising less than 0.5%by weight boron expressed as B₂O₃.
 10. A composite material as claimedin claim 9 and comprising less than 0.1% by weight boron expressed asB₂O₃.
 11. A composite material as claimed in claim 1 in which thealkaline earth silicate fibre is itself adapted for use withoutexcessive shrinkage at temperatures in excess of 1200° C.
 12. Acomposite material as claimed in claim 1 in which the material isobtainable by vacuum forming from a slurry containing the followingingredients in weight %:— Alkaline earth metal silicate fibres 70–85%Colloidal silica (30% SiO₂ by weight) 3–25% Organic binder 1–6% Filler11–20%.
 13. A composite material as claimed in claim 1 comprising:—Alkaline earth metal silicate fibres 70–90% Silica binder from colloidalsilica (30% SiO₂ by weight) 1–10% Organic binder 1–6% Filler 11–20%. 14.A composite material as claimed in claim 13 comprising:— Alkaline earthmetal silicate fibres 77.3–87.2% Silica binder from colloidal silica(30% SiO₂ by weight) 1.2–8.2% Organic binder 3.3–4.7% Filler 12.8–18%.15. A composite material as claimed in claim 1 in which the material isa paper comprising:— Alkaline earth metal silicate fibre 90–95% Organicbinder 5–10% Organic flocculants <1%.
 16. A composite material asclaimed in claim 15 in which the organic binder is an acrylic latex. 17.A composite material as claimed in claim 1 in which the material is amaterial obtained by vacuum forming from a slurry comprising theingredients: Alkaline earth metal silicate fibre 60 parts by weightColloidal silica (30% by weight SiO₂) 12–14 parts by weight Starch 2.5parts by weight based upon the total weight of solids added to theslurry; and in which the colloidal silica has a pH of less than
 8. 18. Acomposite material comprising 4–12% by weight colloidal silica, 3–6.5%starch, balance alkaline earth silicate fibre, to total 100% based onthe weight of composite material.
 19. A composite material as claimed inclaim 18 and comprising 4–9% by weight colloidal silica, 3.5–5% starch,balance alkaline earth silicate fibre, to total 100% based on the weightof composite material.
 20. A composite material as claimed in claim 18comprising about 6% colloidal silica.
 21. A composite material asclaimed in claim 1 in which the material is a material obtainable byvacuum forming from the ingredients:— White water component 50–80% byvolume of 30% solids colloidal silica with 20–50% by volume waterAlkaline earth metal silicate fibre 0.5–4% by weight of fibre,calculated as the weight of the fibre solids per weight of white watercomponent and in which the colloidal silica has a pH of less than
 8. 22.A composite material as claimed in claim 1 in which the material is amaterial obtainable by vacuum forming from the ingredients:— White watercomponent 90–100% by volume of 30% solids colloidal silica with 10–0% byvolume water Alkaline earth metal silicate fibre 2–3% by weight offibre, calculated as the weight of the fibre solids per weight of whitewater component and in which the colloidal silica has a pH of less than8.
 23. A composite material as claimed in claim 21 and which comprises15–30% by weight silica binder formed from colloidal silica, balancefibre.
 24. A composite material as claimed in claim 17 in which thefibre is present in amounts comprising 0.5–5% by weight of the water inthe slurry.
 25. A composite material as claimed in claim 1 in which thematerial is a material obtainable by vacuum forming from the ingredientsWhite water component 65–100% by volume of 40% solids colloidal silicahaving a pH of less than 10 with 35–0% by volume water Alkaline earthmetal silicate fibre 2–3% by weight of fibre, calculated as weight offibre solids per weight of white water component wherein the sodiumcontent of the colloidal silica is below 0.1 wt %.
 26. A compositematerial as claimed in claim 22 and which comprises 15–30% by weightcolloidal silica, balance fibre.